The process by which evolutionary forces act upon standing genetic variation to cause species formation remains one of the greatest unanswered questions in biology. Two primary goals of research on the speciation process are determining the genetic changes underlying reproductive barriers and identifying the evolutionary forces driving their divergence. To test the multitude of hypotheses regarding speciation, it is critical to first identify the genes responsible for species divergence, knowledge that is currently extremely limited as only four genes affecting reproductive isolation have been identified. This proposal focuses on using gene expression assays and transgenics to identify the genetic basis of 1) behavioral isolation preventing successful copulation through characterization of candidate genes that were previously indicated through quantitative trait locus mapping affecting copulation occurrence between Drosophila simulans and D. mauritiana and 2) the formation of sterile offspring through identification of genes responsible for hybrid sterility between D. simulans and its sibling species D. mauritiana and D. sechellia. Information from studies of Drosophila fertility can then potentially be applied to understanding the genetics of human fertility.